Believe it or not there is a UK 'standard' width laid down in law for parking spaces! And it hasn't been changed since it was introduced in the 1950s even though cars have grown so much. Believe it's about 1.8 metres, which is narrower than many cars, esp 4x4s!
So as long as Sainsbury's (or whoever the car park belongs to) have followed this width then no-one would get anywhere trying to prove in law that they hadn't carried out their statutory duty of care. Sorry!